The present invention relates to the field of injection molding, particularly the design of valve gate systems. Referring to FIGS. 1A and 1B, valve gated injection molding systems according to the prior art such as the one, indicated generally at 100, use a reciprocating piston 108 connected to a shutoff pin 112 inside a melt-flow bore 114 to control the flow of molten polymer. The melt flow bore 114, which is the passageway in which the molten polymer travels, is sealed from an unheated clamp plate 102 and parts interior to it by a manifold seal 116. Often, a small chamber 118 separates the manifold seal 116 and the cylinder 110.
The piston 108 is pneumatically moved along its axis 120 by selectively supplying a fluid, preferably air or an inert gas, into the cylinder 110 with the fluid acting on the piston 108. A close gate supply port (not shown) is disposed to be rearward of a rear pressure surface 122 of the piston 108, such that the close gate supply port is in fluid communication with the rear pressure surface 122 of the piston 108. As the piston 108 moves, the associated shutoff pin 112 moves with it, closing the gate 105 and shutting off the flow of molten polymer that flows down the melt-flow bore 114 and into the mold 107.
The piston 108 travels along its axis 120 inside the cylinder 110. Cylinder 110 rests inside the clamp plate 102 with the forward end 109 of the cylinder 110 directly contacting a hot manifold 104. A rear end 111 of the shutoff pin 112 is affixed to the piston 108 and passes through the cylinder 110 through a hole at its forward end. The shutoff pin 112 extends through the hot manifold 104 and into a manifold plate 106. The hot manifold 104 is heated to keep the polymer in its molten state. The forward end 113 of the shutoff pin 112 terminates at a nozzle seal 115 at a forward end of the manifold plate 106 and controls the flow of the molten polymer into the mold. Thus, the gate 105 is formed by the forward end 113 of the shutoff pin 112 and the nozzle seal 115. The gate 105 is closed when the piston 108 is in the forward position and open when the piston 108 is in the rear position. If the gate 105 is open, molten polymer passes through the gate 105 and into the mold cavity 132.
Referring to FIG. 1B, showing a prior art assembly 100 with the piston in the rear position, the piston 108 has a forward pressure surface 124 and a piston stop surface 126 which contacts a cylinder stop surface 128 on the forward wall 117 of the cylinder 110 when the piston is in the forward position. To raise the piston 108, the fluid enters through an open gate supply port 130 that is in fluid communication with the forward pressure surface 124.
As shown in FIGS. 1A and 1B, the piston stop surface 126 is radially inward of the forward pressure surface 124. Since the cylinder 110 is in direct contact with the heated hot manifold 104, thermal energy from the hot manifold 104 passes into the cylinder 110 and, when the piston 108 is in the forward position, then into the front end of the piston 108, creating a temperature gradient between the forward portions of the assembly 100 and the rear portions of the assembly. This heat transfer into the cylinder 108 is undesirable because the piston uses O-rings 119 to maintain the pneumatic pressure on the piston 108 and stop leakage of fluid from one side of the piston to the other. Heat from the hot manifold 104 causes the O-rings 119 to degrade. This causes increased maintenance time and lost production.
Accordingly, it is an object of the present invention to provide a system that reduces the heat transferred from the hot manifold 104 to the piston 108, thereby reducing maintenance on the system and increasing the run time.
Further, it is an object of the present invention to increase the ease of the maintenance of the injection molding systems by providing a system wherein the piston and the shutoff pin may be removed without removing the clamp plate from the hot manifold.